Film-type and sheet-type adhesion-preventing agents comprising biocompatible substances have come to be used in surgical procedures. In the prior art, films employing materials intended for such applications, such as collagen-based materials or gel compositions derived from hyaluronic acid and carboxymethyl cellulose, have been proposed (for example, see Patent Document 1 and Patent Document 2). However, problems have been pointed out, such as this type of film being difficult to handle during surgery and exhibiting low covering properties on the surface of tissues. Meanwhile, development of materials to be used directly in the form of gels has progressed, and use of a carboxymethyl cellulose derivative that enables the formation of a hydrogel having excellent viscoelasticity as a medical gel or adhesion-preventing material has been proposed (for example, see Patent Document 3). Development of some of these gel-like adhesion-preventing materials has already progressed with a view to commercialization. However, the need remains for a gel-like adhesion-preventing material (or agent) having more effective adhesion prevention performance.